first thank you for identifying the pictures here
There is a second page from the Book of Hours hanging over the fire which is obscured by a vase. I am pretty sure that that page hanging there is April https://en.wikipedia.org/wiki/File:Les_Très_Riches_Heures_du_duc_de_Berry_avril.jpg
As you can see the building vaguely on the right and in one shot vaguely the man in the blue garment.
The choice of April could be interpreted in a way as there are theories out there that Jesus was born in April and Peter is stating that the day Jesus was born is unknown.
From the Article:
Then Peter stands in front of the fireplace for a while, and he probably gazes at Nativity when he says "Imagine that: Christ wasn't even born on this day, maybe not even 1,997 years ago. So no one knows for sure when the millennium really begins and ends. Or how much time is left."
A possibility is that Peter is gazing on both pictures - the Nativity painting and on the April Calendar painting.
Was anyone able to identify so far the yellow (wheat?) field painting:
And the painting when Frank is entering Peter's study room?